Air riding seals can be used for providing a seal between relatively rotating parts. In many cases one of the parts will be stationary, but air riding seals may also be used between two rotating parts, which may rotate in the same direction as each other, or in opposite directions. For ease of explanation, parts will be referred to in this specification as a rotating part and a stationary part, but it will be appreciated that the “stationary” part may itself rotate.
A typical air riding seal comprises a runner which is mounted on the rotating part and a stationary element which is mounted on the stationary part. The runner may, for example, be made from a metallic material, and the stationary element may be made from a self-lubricating material such as carbon. The stationary element may also be referred to as a carrier module. Whilst the runner may be mounted on the rotating part, the runner may also be integrally formed in the rotating part. For example, in the case that the rotating part is a shaft, the runner may be e.g. a flange formed in, and extending radially from, the shaft.
The runner and the carrier module have axial sealing faces which are disposed face-to-face. When relative rotation occurs between the rotating and stationary parts, a film of air is drawn between the sealing faces, causing them to lift off from each other so that a cushion of air is formed between them. Consequently, there is no face-to-face contact between the runner and the stationary element once the relative speed of rotation exceeds a threshold value, and sliding friction between the faces is substantially eliminated. The gap between the faces is very small, and effectively prevents flow from one side of the seal to the other.
Air-riding seals may be used in gas turbine engines. For example, the seals may provide sealing between shafts of the engine, rotating at different speeds, or between a rotating shaft and a stationary component. Air riding seals may be employed to seal between regions of the engine containing air at different pressures, or to prevent escape of a liquid, such as lubricating oil, from a region in which it is to be confined.
A conventional air-riding seal arrangement is shown schematically in FIG. 1. Here, the air-riding seal arrangement provides a seal between a first component in the form of a rotating shaft 125 and a second component in the form of a housing 127. The shaft is rotatable about an axis X-X, and it will be appreciated that the seal arrangement is substantially axis-symmetric about this axis.
A runner 129 is mounted on the shaft 125 so that the runner 129 is rotationally fixed with respect to the shaft 125.
A front portion 131 and a rear portion 133 of a carrier module are rotationally fixed with respect to the housing 127, and consequently do not rotate with the shaft 125. The carrier member is biased by a spring formation 135 towards the runner 129.
The runner 129 and the carrier member have oppositely disposed annular sealing faces 137, 139 which, when the shaft 125 is stationary, are in contact with each other under the influence of the spring 135 formation. The sealing face 139 of the runner is provided with formations (not visible) which provide an aerodynamic lifting force when the shaft rotates. The formations may, for example, take the form of spiral grooves in the sealing face. However, such formations are not present in all conventional air-riding seal arrangements, as in many cases the pressure difference across the seal provides sufficient aerodynamic lifting force.
When the shaft 125 rotates about the axis X-X at a speed above a threshold, the aerodynamic lift generated between the sealing faces 137 and 139 causes the carrier module to be displaced away from the sealing face 139 of the runner 129 by a small distance. The resulting gap is filled by a relatively stiff layer of air which not only prevents face-to-face contact between the sealing faces 137, 139, but also prevents flow across the seal, i.e. from a first region B to a second region A. Consequently, the seal arrangement is able to maintain a pressure difference between the regions A and B. It can also prevent the transfer of fluid between the regions A and B. For example, the region A may be a bearing chamber accommodating a lubricated bearing (not shown), with the result that the region A may contain a mist of lubricant droplets in air. It is also possible that lubricant delivery systems, such as jets, may supply lubricant to the region A, for example to supply lubricant to the bearing. The seal arrangement shown in FIG. 1 may thus be able to prevent migration of the lubricant from the region A to the region B.
As well as the primary seal formed between the sealing faces 137, 139, a secondary sealing element 140 such as a PTFE ring seal may be provided to prevent flow through a secondary flow path behind the carrier module.
However, such conventional air-riding seal arrangements can suffer a problem of high-speed touchdown (HSTD) of the sealing faces. High speed touchdown occurs when the two sealing faces contact, rather than maintaining an air-riding state. This contact may generate a large heat input e.g. of many kW, which may cause wear and possibly failure of the faces and of the secondary sealing. Conventional air-riding seal arrangements do not provide any ‘safe-fail’ features, or features which are capable of recovering to an air-riding state from an HSTD event. Thus conventionally, the only way to treat this risk of failure is simply to minimise the probability of an HSTD event occurring. This limits present use of air-riding seals, due to the risk of component failure in use.